1. Field of the Invention
The present invention relates to a laminated balun transformer, and more particularly, to a subminiaturized laminated balun transformer with an improved insertion loss characteristic in a pass band.
2. Description of the Related Art
The term “balun” in balun transformer, which is an abbreviation for balanced to unbalanced, refers to a device composed of a circuit or structure for transforming a balanced signal to an unbalanced signal and vice versa. For example, this is required to perform a transformation between a balanced signal and an unbalanced signal when a device having a balanced input/output stage, such as a mixer or an amplifier, is connected to a device having an unbalanced input/output stage, such as an antenna.
The balun transformer can be implemented by a combination of lumped elements such as R, L, and C elements, or distributed elements such as a microstrip line, a strip line and a transmission line. Recently, with the increasing need for miniaturization of radio communication products widely employing the balun transformer, a laminated balun transformer using low temperature cofired ceramics (LTCC) is being frequently used to reduce the size of the elements.
FIG. 1 is an equivalent circuit showing the basic configuration of a general balun transformer as suggested by Marchand. As shown in this figure, the balun transformer is composed of four conductive lines 14 to 17 each having a length of λ/4 (where, λ is 1/fc (fc is the center frequency of an input/output signal)). Of the four conductive lines 14 and 17, a first line 14 has one end connected to an unbalanced port 11 through which an unbalanced signal having a certain frequency is inputted or outputted. A second line 15 has one end connected to the other end of the first line 14. The other end of the second strip line 15 is opened. In addition, a third line 16 and a fourth line 17 each have one respective one end thereof connected to a ground point and are arranged in parallel with the first line 14 and the second line 15, respectively, to form an electrical coupling therebetween. The other ends of the third and fourth lines 16 and 17 are connected respectively to balanced ports 12 and 13 through which a balance signal is inputted or outputted.
In the above configuration, the first line 14 and the third line 16 form one coupler and the second line 15 and the fourth line 17 form another coupler. With the above configuration, when an unbalanced signal having a certain frequency is inputted to the unbalanced port 11, electromagnetic coupling among the first to fourth lines 14 to 17 is generated, and accordingly, the balanced signal having the same frequency and magnitude as the inputted unbalanced signal, but having a phase which is different by 180 degrees from the phase of the unbalanced signal, is outputted through the balanced ports 12 and 13. On the contrary, when two balanced signals having a certain frequency, the same magnitude, and phases, which are different by 180 degrees from each other, are respectively inputted to the balanced ports 12 and 13, an unbalanced signal having the same frequency as the two inputted balanced signals are outputted through the unbalanced port 11.
A conventional laminated balun transformer having such an equivalent circuit is implemented with a shape as shown in FIG. 2 and an internal structure as shown in FIG. 3.
Referring to FIG. 2, the conventional laminated balun transformer 20 is composed of a rectangular hexahedral dielectric block 21 and a plurality of external electrodes 23 to 28 formed on two opposite sides of the dielectric block 21, each of which is set as a terminal such as an unbalanced terminal, a balanced terminal, or a ground terminal. For example, an external electrode 3 is set as a terminal for non-connection, external electrodes 24 and 27 are set as a terminal for a ground, external electrodes 25 and 28 are set as a terminal for input/output of a balanced signal, and an external electrode 26 is set as a terminal for input/output of an unbalanced signal.
Referring to FIG. 3, the dielectric block 21 is composed of a plurality of dielectric sheets laminated using an LTCC method. On the plurality of laminated dielectric sheets are formed a first ground electrode 30 connected to the external electrodes 24 and 27 for a ground, the first strip line 32 having a length of λ/4 and having one end connected to the external electrode 26 for input/output of the unbalanced signal, the third strip line 33 formed in parallel with the first strip line 32, having a length of λ/4 and having both ends connected respectively to the external electrode 25 for input/output of the balanced signal and the external electrode 27 for a ground, a second ground electrode 35 connected to the external electrodes 24 and 27 for a ground, the second strip line 37 having a length of λ/4 and having one end connected to the first strip line 32 via the external electrode 23 and the other end opened, the fourth strip line 38 formed in parallel with the second strip line 37 and having both ends connected respectively to the external electrode 27 for a ground and the external electrode 28 for input/output of the balanced signal, and a third ground electrode 40 connected to the external electrodes 24 and 27 for a ground, sequentially in a downward direction.
Reference numerals 31, 34, 36 and 39, which are not described, denote lead electrodes for connecting the first to fourth strip lines 32, 33, 37 and 38 to respective external electrodes 23 to 28.
As described above, the laminated balun transformer is miniaturized by vertically laminating four λ/4 strip lines. However, recently, with the increased need for a subminiaturized balun transformer, much study has been concentrated on the subminiaturization of the balun transformer while maintaining or improving its basic properties.